The invention is directed to a process for the production of vanadium carbide layers (or coatings) on iron and iron alloys having a carbon content of at least 0.1 weight % by treating the workpiece of the iron or iron alloy at 800.degree. to 1100.degree. C. in a salt bath containing 1 to 30 weight % vanadium or ferrovanadium powder.
Vanadium carbide coatings are distinguished by a great hardness, resistance to wear and good resistance to oxidation and corrosion. Therefore there have been efforts to apply such coatings to workpieces made of iron and iron alloys, especially to tool parts and machine parts having high requirements, such as e.g., drawing tools, stamping tools, cutting tools, or nozzles.
There are already known numerous processes for the production of wear resistant vanadium carbide coatings. Thus German OS No. 2053063 describbes a process for forming a carbide coating of an element of the Va group of the periodic system of elements on the surface of metal objects. For this purpose, there is used a molten salt bath which contains boric acid or a borate and a metal powder of an element of the Va group. Hereby the metal forming the coating is partially dissolved in the very corrosive borate melt and in this manner is transported to the surface of the work material.
A modification of this process is described in German OS No. 2322159 in which case the metal forming the carbide layer is anodically dissolved in the melt.
In German OS No. 2322157 for the production of wear resistant carbide coatings the carbide forming metal is deposited cathodically on the workpiece from a borate melt.
Furthermore, there is described in German OS No. 2819856 a process in which the metal forming the coating is produced by reducing tthe corresponding metal oxide by means of a boron containing material such as ferroboron or boron carbide.
All of these known processes have a number of disadvantages. A common characteristic of the known processes is the use of boron oxide or borates as an essential constituent of the melts Borate or boron oxide containing melts, however, are extraordinarily corrosive at the high temperature of 800.degree.-1100.degree. C. necessary for the treatment. This frequently leads to noticeable attacks on the crucible material or on the work material being treated which sometimes is manifested in considerable surface roughness. Furthermore these melts even at high temperatures are extremely viscous. Because of the high viscosity of the boron containing melts, there can occur a nonuniform temperature distribution in the bath. To be sure the viscosity of these melts can be lowered somewhat by the addition of alkali halides but the viscosity producible thereby is still very high. Through this there is also carried out a considerable amount of the bath with the parts being treated whereby there occur high losses of salt. Furthermore, the melt adhering to the cooled parts is only removed with extraordinary difficulty because of its glass-like nature.
The known electrolytic process causes a larger expense for the process, such as e.g., exact bath controls and maintaining the current density constant and causes additional costs. In the process according to German OS No. 2819859, e.g. it is necessary to keep a very specific ratio of metal oxide to boron since with an excess of boron no carbide layer is formed but instead a boride layer. With a deficiency of boron likewise no carbide layer is formed since the reducing action is not sufficient.
Therefore it was the problem of the present invention to find a process for the production of vanadium carbide coatings on iron and iron alloys, e.g., steel, having a carbon content of at least 0.1 weight % by treatment of the workpiece at 800.degree. to 1100.degree. C. in a salt bath containing 1-30 weight % vanadium or ferrovanadium powder without the use of current and with a lower viscosity, little corrosive salt melt.